This invention relates to an apparatus for driving a brushless motor used in, for example, a video tape recorder (VTR).
Brushless DC motors are most frequently used now as capstan drive motors or cylinder drive motors in video tape recorders. In such a brushless DC motor, a permanent magnet is used as its rotor, and a plurality of armature coils of different phases, for example, three phases are disposed in the stator. The principle of electromagnetic induction between the current supplied to the stator coils and the magnetic field generated by the permanent magnet forming the rotor is utilized to produce the rotation torque rotating the rotor. In the motor, the stator coils of the individual phases are selectively energized depending on the angular position of rotation of the rotor to continuously develop the rotation torque.
The structure and operation of a three-phase brushless DC motor which is a most typical example of the brushless DC motor will be described before describing the present invention in detail.
A prior art, three-phase brushless DC motor of the bipolar drive type and a driving circuit therefor are shown in FIG. 1. In the bipolar drive type of brushless DC motor, current is supplied to its stator coils in two directions. Referring to FIG. 1, the brushless DC motor includes a permanent-magnet rotor 1, and three-phase stator coils L.sub.1, L.sub.2 and L.sub.3. A rotor position detector 11 detects the instant angular position of the rotating rotor 1, and a signal generator 12 generates a signal to selectively drive a plurality of transistors 4 to 9 in response to the output of the rotor position detector 11. The transistors 4 to 9 are connected at their collectors to a common power source 10. An upper one and a lower one of the transistors 4 to 9 in FIG. 1 are simultaneously turned on so that the current from the power source 10 is supplied through the conducting transistors to two of the three stator coils L.sub.1 to L.sub.3 for a predetermined period of time. The stator coils to which the current is supplied are determined by the transistors which are turned on, and the signal generator 12 generates the transistor drive signal at predetermined timing determined by the angular position of rotation of the rotor 1 so as to drive corresponding ones of the transistors 4 to 9. The quantity of current supplied to the individual stator coils L.sub.1 to L.sub.3 is determined on the basis of the desired rotation speed of the motor.
In such a prior art brushless DC motor, the density of the magnetic flux generated from the permanent magnet and crossing the coils changes substantially in a sinusoidal fashion from the aspect of the electrical angle. Consequently, the rotational force generated by the electromagnetic induction changes also depending on the angular position of rotation of the rotor 1. Therefore, the torque causing the rotation of the rotor 1 of the motor includes necessarily a torque ripple, that is, a variation of the rotation speed. Appearance of such a torque ripple has been an especially serious problem for a motor such as a VTR motor required to rotate accurately at a constant speed, and such a problem has necessarily occurred in a polyphase brushless motor.
The mechanism of occurrence of such a turque ripple will now be described with reference to FIGS. 2 and 3.
FIG. 2 shows, in a developed view, the relation between the rotor 1 and the stator coils L.sub.1 to L.sub.3 of the three-phase brushless DC motor shown in FIG. 1. For ease of explanation, it is supposed that the rotor 1 is held stationary, and the stator coils L.sub.1 to L.sub.3 are movable, and, thus, the relation is contrary to the practical case. Referring to FIG. 2, reference numerals 2 and 3 designate yokes on the rotor side and stator side respectively, and the stator coils L.sub.1 to L.sub.3 cross the lines of magnetic flux emanating from the adjacent S and N poles of the permanent magnet and passing through the yokes 2 and 3. In each of the stator coils L.sub.1 to L.sub.3, the black dot symbol indicates the direction of current flowing perpendicularly toward the front side of the drawing sheet, while the symbol X indicates the direction of current flowing perpendicularly toward the back side of the drawing sheet. It is supposed now that the stator coils are movable. Therefore, FIG. 2 illustrates how the stator coils L.sub.1 to L.sub.3 move relative to the rotor 1 with lapse of time from t.sub.1 to t.sub.2, t.sub.2 to t.sub.3, and t.sub.3 to t.sub.4. As described already, it is supposed that the stator coils L.sub.1 to L.sub.3 move relative to the rotor 1 for conveniences of explanation, although actually the rotor 1 rotates relative to the stator coils L.sub.1 to L.sub.3. Therefore, the stator coils L.sub.1 to L.sub.3 move rightward in FIG. 2, thereby producing a positive torque to cause rotation of the motor. In other words, it can be said, on the contrary, that the stator coils L.sub.1 to L.sub.3 are held stationary, and the positive torque acts on the rotor 1 to cause leftward movement of the rotor 1.
The magnetic field established by the permanent magnet is distributed substantially in a sinusoidal fashion as shown in FIG. 2. In FIG. 2, the horizontal axis represents the electrical angle .theta., and the vertical axis represents the magnetic flux density .phi.. Each of the stator coils L.sub.1, L.sub.2 and L.sub.3 is wound to cover an electrical angle of 180.degree. (that is, the width of any one of the N or S poles of the permanent magnet), and these stator coils L.sub.1, L.sub.2 and L.sub.3 are disposed with a phase difference of 120.degree. in electrical angle therebetween.
FIG. 3 shows how curents I.sub.1 to I.sub.3 supplied to the respective stator coils L.sub.1 to L.sub.3, torques T.sub.1 to T.sub.3 produced as a result of the supply of the respective currents I.sub.1 to I.sub.3, and the composite torque T.sub.0 (=T.sub.1 +T.sub.2 +T.sub.3) change relative to time in the apparatus shown in FIG. 1.
Suppose now that the motor drive currents I.sub.1 and I.sub.2 are supplied to the respective stator coils L.sub.1 and L.sub.2 at time t.sub.1 at which the leading end of the stator coil L.sub.1 takes the position of the electrical angle .theta..sub.1 shown in FIG. 2. That is, suppose that a predetermined base drive current is supplied to each of the transistors 6 and 8 from the signal generator 12. Then, the transistors 6 and 8 are turned on in response to the supply of the predetermined quantity of base drive current to each of their bases, and the predetermined current flows through the route which is traced from the power source 10.fwdarw.transistor 6 .fwdarw.coil L.sub.1 .fwdarw.coil L.sub.2 .fwdarw.transistor 8 to the ground. At this time, the remaining transistors are in their cut-off state. Then, when the leading end of the stator coil L.sub.1 reaches the position of the electrical angle .theta..sub.2 at time t.sub.2, the stator coil L.sub.2 is de-energized, and the stator coil L.sub.3 is energized in turn. That is, the transistors 6 and 7 are now turned on, and the predetermined current flows through the route which is traced from the power source 10.fwdarw.transistor 6.fwdarw.coil L.sub.1 .fwdarw.coil L.sub.3 .fwdarw.transistor 7 to the ground. Then, when the leading end of the stator coil L.sub.1 reaches the position of the electrical angle .theta..sub.3 at time t.sub.3, the transistors 5 and 7 are now turned on while turning off the remaining transistors, and the predetermined current is supplied to the stator coils L.sub.2 and L.sub.3. In the manner above described, the transistors are selectively turned on and off under control of the signal from the signal generator 12 each time a predetermined positional relationship is established between the stator coils and the rotor, and the predetermined current is supplied to the corresponding stator coils to attain continuous rotation of the motor. In the illustrated example, the current supply to the stator coils is changed over at the angular interval of the electrical angle of 60.degree.. Thus, there are six modes of coil energization, and one cycle of these modes causes one complete revolution (360.degree.) of the motor. For the purpose of detection of the positional relationship between the stator coils and the rotor, a known rotation position detector such as that using a Hall element can be utilized. Also, the electrical angle between the positions .theta..sub.1 and .theta..sub.3 supplying the current to the stator coils is preferably selected to cover a range of 120.degree. around the position .theta..sub.2 where the magnetic flux density is maximum, in order to achieve a high efficiency.
The torque T.sub.1 shown in FIG. 3 is produced by the coaction of the current I.sub.1 supplied to the stator coil L.sub.1 and the magnetic flux generated from the rotor 1 in the period from time t.sub.1 to time t.sub.2, and it will be seen that the magnitude of this torque T.sub.1 depends upon the density .phi..sub.1 of the magnetic flux crossed by the leading end of the coil L.sub.1. In FIG. 3, I.sub.0 designates the current value corresponding substantially to the predetermined rotation speed of the motor. Also, the torque T.sub.2 shown in FIG. 3 is produced by the coaction of the current I.sub.2 supplied to the stator coil L.sub.2 and the magnetic flux generated from the rotor 1 in the period from time t.sub.1 to time t.sub.2. (In this case, the density .phi..sub.2 of the magnetic flux crossed by the leading end of the stator coil L.sub.2 is phase-shifted by 120.degree. relative to .phi..sub.1). In this period, the current I.sub.3 supplied to the stator coil L.sub.3 is zero, and the torque T.sub.3 is also zero.
In the period from time t.sub.2 to time t.sub.3, the transistors are switched over to de-energize the stator coil L.sub.2 and energize the stator coil L.sub.3 thereby producing the torques T.sub.1 and T.sub.3. In the period from time t.sub.3 to time t.sub.4, the transistors are switched over to de-energize the stator coil L.sub.1 and energize the stator coil L.sub.2 thereby producing the torques T.sub.2 and T.sub.3.
Although the direction of the current supplied to the stator coil L.sub.1 is reversed at time t.sub.4, the direction of the magnetic flux crossed by the leading end of the coil L.sub.1 at that time is also reversed as shown in FIG. 2. Consequently, the direction of the produced torque T.sub.1 remains positive. The currents I.sub.1, I.sub.2 and I.sub.3 supplied to the respective stator coils L.sub.1, L.sub.2 and L.sub.3 produce a composite torque T.sub.0 =T.sub.1 +T.sub.2 +T.sub.3.However, in this composite torque T.sub.0 imparted to the rotor 1, a torque variation occurs at an angular interval of 60.degree. as shown in the lowest portion of FIG. 3. Therefore, the prior art brushless DC motor has been defective in that a flywheel must be used to absorb the variation of the rotation speed of the motor attributable to such a torque ripple. Also, when the motor is driven intermittently at a period shorter than the period of the torque ripple (as, for example, in the case of the so-called fine-slow playback mode in which a tape in a VTR is driven intermittently at the standard speed), there has been such a problem that the desired intermittent drive with high accuracy cannot be achieved.
Further, Japanese Patent Application Laid-open No. 55-79694 filed in Japan on Dec. 12, 1978 by Tokyo Shibaura Electric Co., Ltd. discloses a method for driving a brushless motor with a drive current having a phase opposite to that of the aforementioned torque variation in order to compensate the torque variation in the motor. According to the disclosed method, the relation between the angular position of rotation of the motor and the associated torque variation is stored previously in a memory, and the amount of the torque variation corresponding to the detected angular position of rotation of the motor is read out from the memory, so that, on the basis of the read-out amount of the torque variation, the motor drive current can be suitably controlled by a microcomputer. Therefore, the disclosed method has been defective in that a memory and a microcomputer resulting in a cost increase are essentially required in addition to the necessity for previously finding the relation between the torque variation and the angular position of rotation of the motor.